Optical disc drive apparatus, method for measuring tilt of an optical disc, and method for correcting tilt of an optical disc

ABSTRACT

Tilt (θ(r, φ)) is measured in a measuring location (P(r, φ) optical disc ( 2 ). A pivotable objective lens ( 34 ) is brought to a first focus measuring location such as to focus a light beam ( 32 ) in a first anchor point (P 1 (r−Δr 1 , φ))having the same angular coordinate φ as said measuring location (P(r,φ)) and having a small radial distance Δr 1  from said measuring location. The objective lens is brought to a second focus measuring location such as to focus the light beam in a second anchor point (P 2 (r+Δr 2 , φ)) having the same angular coordinate φ as said measuring location and having a small radial distance Δr 2  from said measuring location, wherein said first and second anchor pints are located on opposite sides of said measuring location. Tilt ii said measuring location is calculated from the coordinates of said two focus measuring locations of said objective lens.

The present invention relates in general to disc drive systems forstoring information onto a disc-shaped storage medium or readinginformation from such disc-shaped storage medium, where the disc isrotated and a write/read head is moved radially with respect to therotating disc. The present invention is applicable in the case ofoptical or magneto-optical disc systems. Hereinafter, the wording“optical disc system” will be used, but it is to be understood that thiswording is intended to also cover magneto-optical disc systems.

As is commonly known, an optical storage disc comprises at least onetrack, either in the form of a continuous spiral or in the form ofmultiple concentric circles, of storage space where information may bestored. Optical discs may be read-only type, where information isrecorded during manufacture, which data can only be read by a user. Theoptical storage disc may also be a writable type, where information maybe stored by a user. For writing information in the storage space of theoptical storage disc, or for reading information from the disc, anoptical disc drive comprises, on the one hand, rotating means forreceiving and rotating an optical disc, and on the other hand opticalmeans for generating an optical beam, typically a laser beam, and forscanning the storage track with said laser beam. Since the technology ofoptical discs in general, the way in which information can be stored inan optical disc, and the way in which optical data can be read from anoptical disc, is commonly known, it is not necessary here to describethis technology in more detail.

For rotating the optical disc, an optical disc drive typically comprisesa motor, which drives a hub engaging a central portion of the opticaldisc. Usually, the motor is implemented as a spindle motor, and themotor-driven hub may be arranged directly on the spindle axle of themotor.

For optically scanning the rotating disc, an optical disc drivecomprises a light beam generator device (typically a laser diode), anobjective lens for focussing the light beam in a focal spot on the disc,and an optical detector for receiving the reflected light reflected fromthe disc and for generating an electrical detector output signal.

During operation, the light beam should remain focussed on the disc. Tothis end, the objective lens is arranged axially displaceable, and theoptical disc drive comprises focal actuator means for controlling theaxial position of the objective lens. Further, the focal spot shouldremain aligned with a track or should be capable of being positionedwith respect to a new track. To this end, at least the objective lens ismounted radially displaceable, and the optical disc drive comprisesradial actuator means for controlling the radial position of theobjective lens.

More particularly, the optical disc drive comprises a sledge which isdisplaceably guided with respect to a disc drive frame, which frame alsocarries the spindle motor for rotating the disc. The travel course ofthe sledge is arranged substantially radially with respect to the disc,and the sledge can be displaced over a range substantially correspondingto the range from inner track radius to outer track radius. Said radialactuator means comprise a controllable sledge drive, for instancecomprising a linear motor, a stepper motor, or a worm gear motor.

The displacement of the sledge is intended for roughly positioning theoptical lens. For fine-tuning the position of the optical lens, theoptical disc drive comprises a lens platform which carries the objectivelens and which is displaceably mounted with respect to said sledge. Thedisplacement range of the platform with respect to the sledge isrelatively small, but the positioning accuracy of the platform withrespect to the sledge is larger than the positioning accuracy of thesledge with respect to the frame.

In many disc drives, the orientation of the objective lens is fixed,i.e. its axis is directed parallel to the rotation axis of the disc. Insome disc drives, the objective lens is pivotably mounted, such that itsaxis can make an angle with the rotation axis of the disc. Usually, thisis implemented by making the platform pivotable with respect to thesledge.

It is a general desire to increase the storage capacity of a recordmedium. One way of fulfilling this desire is to increase the storagedensity; to this end, optical scanning systems have been developedwherein the objective lens has a relatively high numerical aperture(NA). One problem involved in such optical systems is the increasedsensitivity to tilt of the optical disc. Tilt of the optical disc can bedefined as a situation where the storage layer of the optical disc, atthe location of the focal spot, is not exactly perpendicular to therotation axis. Tilt can be caused by the optical disc being tilted as awhole, but is usually caused by the optical disc being warped, and as aconsequence the amount of tilt depends on the location on disc.

Thus, there is a demand for a compensation system, and for a method tomeasure tilt.

It is possible to measure the tilt with a separate tilt sensor. However,such solution would involve additional hardware and increased costs.

The present invention proposes a tilt measuring method which does notneed a separate tilt sensor. For measuring the tilt in a certainlocation of the optical disc, this measuring location beingcharacterized by a certain radial coordinate and a certain angularcoordinate, the optical beam is used to obtain the coordinates of twolocations on opposite sites of the measuring location, located along thesame radial line as the measuring location, i.e. having the same angularcoordinate as the measuring location. The tilt angle at the measuringlocation is calculated from the relative axial distance and the relativeradial distance between said two opposite locations.

The coordinates of said locations on opposite sites of the measuringlocation, which will be indicated as “anchor locations”, are obtained byfocusing the optical beam in such anchor locations. In a known method inaccordance with this principle, the focus point of the optical beam isbrought to such anchor locations by displacing the objective lensradially and axially. It is a main objective of the present invention toprovide an alternative to this known method.

A disadvantage of said known method is associated with the fact that theobjective lens is brought to a different radial position. Thus, if themethod is performed while the apparatus is reading or writing a track,the objective lens necessarily looses its original track position andmust be radially displaced back to its original radial position beforethe apparatus can resume operation. It is a further objective of thepresent invention to overcome this disadvantage.

The present invention is based on the understanding that the radialcoordinate of the focal point of the optical beam is displaced radiallywhen the objective lens is pivoted. Based on this understanding, in anoptical disc drive of a type where the optical lens can be pivoted, themeasuring method according to the present invention is performed bydisplacing the optical lens in axial direction and in pivot direction.In one embodiment, the method is performed by first focusing the opticalbeam on the measuring location, moving the objective lens axiallytowards the optical disc, and pivoting the objective lens into twodirections to obtain two focal locations. In an alternative embodiment,the optical beam is first focused at the measuring location, then theobjective lens is pivoted into one direction followed by the objectivelens being axially displaced towards the optical disc.

Further, a disadvantage of the known method is that it is restricted tofinding a value for the tilt in a certain location. This value will beused as input in a device for correcting the tilt. In an optical discdrive of a type where the optical lens can be pivoted, the correctiveaction may consist in giving the optical lens a suitable pivot offset,i.e. pivoting the optical lens over a suitable pivot angle. One way offinding the appropriate pivot offset is to calculate a pivot angle onthe basis of the measured tilt. Such calculation may, however, introduceerrors, especially if circumstances like temperature change, and merelycalculating a pivot angle does not offer any feedback mechanism whichenables a check as to whether the calculated pivot angle is the optimalangle.

The present invention also aims to provide a solution to this problem.More particularly, the present invention aims to provide a method forsetting the pivot offset of the objective lens such that a tilt of thedisc is corrected as good as possible, without the need to know the sizeof the tilt. In one embodiment, the method for setting the pivot offsetof the objective lens is performed by setting the pivot offset to acertain value, then focusing the optical beam on the measuring location,moving the objective lens axially towards the optical disc, and pivotingthe objective lens into two directions to obtain two focal locations. Ifthe two pivot angles are equal, the pivot offset is assumed to becorrect. If not, the pivot offset is amended, and the above steps arerepeated.

In an alternative embodiment, the pivot offset of the objective lens isset to a certain value, the optical beam is focused at the measuringlocation, then the objective lens is pivoted into one direction followedby the objective lens being axially displaced towards the optical disc.Then, the objective lens is pivoted into the opposite direction over thesame angle followed by the objective lens being axially displacedtowards the optical disc. If the two axial displacements are equal, thepivot offset is assumed to be correct. If not, the pivot offset isamended, and the above steps are repeated.

These and other aspects, features and advantages of the presentinvention will be further explained by the following description withreference to the drawings, in which same reference numerals indicatesame or similar parts, and in which:

FIG. 1 schematically shows a disc drive apparatus;

FIGS. 2-4 schematically illustrate embodiments of the measuring methodof the present invention;

FIGS. 5A-B are flow charts illustrating steps of disc tilt measuringmethods of the present invention;

FIGS. 6A-B are flow charts illustrating steps of pivot angle settingmethods of the present invention.

FIG. 1 schematically illustrates an optical disc drive 1, suitable forstoring information on or reading information from an optical disc 2.The disc drive apparatus 1 comprises an apparatus frame 3. For rotatingthe disc 2, the disc drive apparatus 1 comprises a motor 4 fixed to theframe 3, defining a rotation axis 5. For receiving and holding the disc2, the disc drive apparatus 1 may comprise a turntable or clamping hub6, which in the case of a spindle motor 4 is mounted on the spindle axle7 of the motor 4.

The disc drive apparatus 1 further comprises a displaceable sledge 10,which is displaceably guided in the radial direction of the disc 2, i.e.in a direction substantially perpendicular to the rotation axis 5, byguiding means not shown for the sake of clarity. A radial sledgeactuator, designed for regulating the radial position of the sledge 10with respect to the apparatus frame 3, is schematically indicated at 11.Since radial sledge actuators are known per se, while the presentinvention does not relate to the design and functioning of such radialsledge actuator, it is not necessary here to discuss the design andfunctioning of a radial sledge actuator in great detail.

In the following, the rotation axis 5 will be taken as Z-axis.Associated with the apparatus 1, a rectangular coordinate system XYZwill be used, wherein the displacement direction of the sledge 10 willbe taken as X-axis, whereas an Y-axis is defined perpendicular to theX-axis and the Z-axis. Associated with the disc 2, a polar coordinatesystem r, φ will be used.

The disc drive apparatus 1 further comprises a displaceable platform 20,which is displaceable in the radial direction of the disc 2 with respectto the sledge 10, and which is displaceably mounted with respect to thesledge 10 by mounting means not shown for the sake of clarity. A radialplatform actuator arranged for radially displacing the platform 20 withrespect to the sledge 10, is indicated at 21. Since such radial platformactuators are known per se, while further the design and operation ofsuch radial platform actuator is no subject of the present invention, itis not necessary here to discuss the design and operation of such radialplatform actuator in great detail.

The disc drive apparatus 1 further comprises an optical system 30 forscanning tracks (not shown) of the disc 2 by an optical beam. Morespecifically, the optical system 30 comprises a light beam generatingmeans 31, typically a laser such as a laser diode, which may be mountedwith respect to the apparatus frame 3 or the sledge 10, and which isarranged to generate a light beam 32 a which passes a beam splitter 33and an objective lens 34 carried by the platform 20. The objective lens34 focuses the light beam 32 b on the disc 2. For achieving anmaintaining a correct focusing of the light beam 32 b, exactly on thedesired location of the disc 2, said platform 20 is also mounted axiallydisplaceable (Z-direction) with respect to the sledge 10, while furtherthe disc drive apparatus 1 also comprises an axial platform actuator 37arranged for axially displacing the platform 20 with respect to thesledge 10. Since such axial platform actuators are known per se, whilefurther the design and operation of such axial platform actuator is nosubject of the present invention, it is not necessary here to discussthe design and operation of such axial platform actuator in greatdetail.

Said objective lens is mounted pivotably with respect to the apparatusframe 3. In an exemplary embodiment, this is achieved by said platform20 being mounted pivotably with respect to the sledge 10 by mountingmeans not shown for the sake of clarity. The platform 20 can pivot abouta pivot axis 40 which is directed parallel to the Y-axis, such that anoptical axis 36 of the objective lens 34 is always located in theXZ-plane. Preferably, as illustrated, the pivot axis 40 coincides withthe optical centre of the objective lens 34. A pivot angle (ψ) will bedefined as the angle between the Z-axis and the optical axis 36 of theobjective lens 34. Since pivotably mounted platforms are known per se,while further the design and operation of such pivotably mountedplatforms is no subject of the present invention, it is not necessaryhere to discuss the design and operation of such pivotably mountedplatform in great detail.

Further, the disc drive apparatus 1 also comprises a pivot platformactuator 41 arranged for pivoting the platform 20 with respect to thesledge 10. Since such pivot platform actuators are known per se, whilefurther the design and operation of such pivot platform actuator is nosubject of the present invention, it is not necessary here to discussthe design and operation of such pivot platform actuator in greatdetail.

The disc drive apparatus 1 further comprises a control unit 90 having afirst output 90 a connected to a control input of the motor 4, having asecond output 90 b coupled to a control input of the radial sledgeactuator 11, having a third output 90 c coupled to a control input ofthe radial platform actuator 21, having a fourth output 90 d coupled toa control input of the axial platform actuator 37, and having a fifthoutput 90 e coupled to a control input of the pivot platform actuator41. The control unit 90 is designed to generate at its first output 90 aa control signal S_(CM) for controlling the motor 4, to generate at itssecond control output 90 b a control signal S_(CS) for controlling thesledge actuator 11, to generate at its third output 90 c a controlsignal S_(CPr) for controlling the radial platform actuator 21, togenerate at its fourth output 90 d a control signal S_(CPa) forcontrolling the axial platform actuator 37, and to generate at its fifthoutput 90 e a control signal S_(CPp) for controlling the pivot platformactuator 41.

The light beam 32 b reflects from the disc 2 (reflected light beam 32 c)and passes the objective lens 34 and the beam splitter 33 (beam 32 d) toreach an optical detector 35 mounted with respect to the sledge 10. Thecontrol unit 90 further has a read signal input 90 f for receiving aread signal S_(R) from the optical detector 35. As will be clear to aperson skilled in the art without needing further explanation, the readsignal S_(R) contains information relating to the fact whether or notthe optical beam 32 b is accurately focused on the optical disc 2. Moreparticularly, a focal error signal (FES) can be derived from the readsignal S_(R).

A point P is shown, having polar coordinates r and φ. In an ideal case,the normal to the surface in point P(r,φ)) is exactly parallel to theZ-axis, but in the case where the disc 2 has a warped surface, as shown,the normal in point P(r,φ)) makes an angle θ(r,φ) with the Z-axis. Thisangle θ(r,φ) will be taken as measure of the tilt in point P(r,φ). Thetilt may vary over the surface of the disc, in other words the tiltθ(r,φ) is a function of radial coordinate r and angular coordinate φ. Itis desirable to know θ(r,φ) for an entire disc, or at least for thatpart of the disc which is to be accessed.

FIG. 2 schematically illustrates the basic principle underlying themeasuring method proposed by the present invention. On a radial linethrough point P(r,φ), two points P1 and P2 of the disc 2 are shown onopposite sides of point P. In the polar coordinate system of the disc,these two points P1 and P2 have coordinates (r₁,φ) and (r₂,φ),respectively. In the rectangular coordinate system of the disc drive 1,the points P, P1 and P2 have coordinates (X0,Y0,Z0), (X1,Y0,Z1),(X2,Y0,Z2), respectively. It can easily be seen that the tilt θ(r,φ) inpoint P(r,φ) can be expressed by the following formula:tan θ(r,φ)=(Z 1−Z 2)/(X 2−X 1)In the following, the location P(r,φ) where the tilt is to be measuredwill be indicated as measuring location. Said two points P1 and P2 onopposite sides of the measuring location will be indicated as anchorpoints.

The present invention is based on finding, with respect to a measuringlocation, two anchor points P1 and P2, and measuring the relative radialdistance X2−X1 and the relative axial distance Z1−Z2 between theseanchor points P1 and P2. As soon as this information is available, thetilt θ at the measuring location can be calculated.

FIG. 3 illustrates a first implementation of the above-mentioned basicmeasuring principle.

In a first step, the objective lens 34 is brought to an initial focusposition (indicated as 34 ¹ in the drawing) where the focal spot of thelight beam 32 coincides with measuring location P(r,φ). The coordinates(z,y,z,ψ) of the objective lens 34 in this initial focus position aredefined as (x₀,0,z₀,ψ₀) (since the objective lens can not move in theY-direction, its Y-coordinate is always constant and maybe taken aszero). In the XYZ-coordinate system, the coordinates of the measuringlocation P(r,φ) are (x₀,0,z_(p)). The distance z_(p)-z₀ corresponds tothe focal distance f of the objective lens, which is taken to beconstant.

In a second step, the objective lens 34 is pivoted over a first angleΔψ₁ towards smaller radius, to reach a position (34 ²) havingcoordinates (x₀,0,z₀,ψ₀−Δψ₁). By such step, the beam is now out offocus. Focus is regained by, in a third step, displacing the objectivelens 34 axially until the control unit 90 finds that the optical beam 32is again focused on the disc 2. Depending on the amount of tilt of thedisc 2, this will require a certain axial displacement Δz₁, so that theobjective lens 34 has reached a first focus measuring position (34 ³)having coordinates (x₀,0,z₀+Δz₁,ψ₀−Δψ₁).

This implies that the first anchor point P1 has coordinates(x₀−f·sin(Δψ₁),0,z₀+f·cos(Δψ₁)+Δz₁). The control unit 90 will store themagnitude Δz₁ of this axial displacement in a memory.

In a fourth step, the objective lens 34 is brought back to the initialfocus position (x₀,0,z₀,ψ₀).

In a fifth step, the objective lens 34 is pivoted over a second angleΔψ₂ towards larger radius, to reach a position (34 ⁴) having coordinates(x₀,0,z₀,ψ₀+Δψ₂). In a sixth step, the objective lens 34 is displacedaxially until the control unit 90 finds that the optical beam 32 isagain focused on the disc 2. Depending on the amount of tilt of the disc2, this will require a certain axial displacement Δz₂, so that theobjective lens 34 has reached a second focus measuring position (34 ⁵)having coordinates (x₀,0,z₀−Δz₂,ψ₀+Δψ₂). This implies that the secondanchor point P2 has coordinates (x₀+f·sin(Δψ₂),0,z₀+f·cos(Δψ₂)−Δz₂). Thecontrol unit 90 will also store the magnitude Δz₂ of this axialdisplacement in a memory.

The control unit 90 can now calculate the tilt θ(r,φ) of the measuringlocation P(r,φ) of the disc 2 from the values of f, Δψ₁, Δz₁, Δψ₂, Δz₂,according to the general formula:tan θ(r,φ)=(f·cos(Δψ₁)+Δz ₁−(f·cos(Δψ₂)−Δz ₂))/(f·sin(Δψ₁)+f·sin(Δψ₂))

Preferably, although not essentially, the first angle Δψ₁ is equal tothe second angle Δψ₂. In that case, the above formula simplifies asfollows:tan θ(r,φ)=(Δz ₁ +Δz ₂)/(2f·sin(Δψ₁))For small values of Δz₁ and Δz₂, the above formula can be approximatedas follows:tan θ(r,φ)=(Δz ₁ +Δz ₂)/(2f·Δψ ₁)

It is noted that the fourth step may be omitted.

It is further noted that the fifth and sixth steps may be taken beforethe second and third steps, so as to first reach the anchor location P₂at larger radius and then reach the anchor location P₁ at smallerradius.

FIG. 4 illustrates a second implementation of the above-mentioned basicmeasuring principle.

In a first step, the objective lens 34 is brought to an initial focusposition (indicated as 34 ¹ in the drawing) where the focal spot of thelight beam 32 coincides with measuring location P(r,φ). The coordinates(z,y,z,ψ) of the objective lens 34 in this initial focus position aredefined as (x₀,0,z₀,ψ₀) (since the objective lens can not move in theY-direction, its Y-coordinate is always constant and may be taken aszero). In the XYZ-coordinate system, the coordinates of the measuringlocation P(r,φ) are (x₀,0,z_(p)). The distance z_(p)−z₀ corresponds tothe focal distance f of the objective lens, which is taken to beconstant.

In a second step, the objective lens 34 is axially displaced over adistance Δz towards the disc, to reach a position (34 ²) havingcoordinates (x₀,0,z₀+Δz,ψ₀). By such step, the beam is now out of focus.

In a third step, the objective lens 34 is pivoted towards smaller radiusuntil the control unit 90 finds that the optical beam 32 is againfocused on the disc 2. Depending on the amount of tilt of the disc 2,this will require a certain first pivot angle Δψ₁, so that the objectivelens 34 has reached a first focus measuring position (34 ³) havingcoordinates (x₀,0,z₀+Δz,ψ₀−Δψ₁).

This implies that the first anchor point P1 has coordinates(x₀−f·sin(Δψ₁),0,z₀+f·cos(Δψ₁)). The control unit 90 will store themagnitude Δψ₁ of this first pivot angle in a memory.

In a fourth step, the objective lens 34 is pivoted towards larger radiusuntil the control unit 90 finds that the optical beam 32 is againfocused on the disc 2 in a second focus measuring position (34 ⁴) havingcoordinates (x₀,0,z₀+Δz,ψ₀+Δψ₂).

This implies that the second anchor point P2 has coordinates(x₀+f·sin(Δψ₂),0,z₀+f·cos(Δψ₂)). The control unit 90 will also store themagnitude Δψ₂ of this second pivot angle in a memory.

The control unit 90 can now calculate the tilt θ(r,φ) of the measuringlocation P(r,φ) of the disc 2 from the values of Δψ₁, Δψ₂, Δz, accordingto the general formula:tan θ(r,φ)=(cos(Δψ₁)−cos(Δψ₂))/(sin(Δψ₁)+sin(Δψ₂))

For small values of Δψ₁ and Δψ₂, the above formula can be approximatedas follows:tan θ(r,φ)=(Δψ₁−Δψ₂)/2

It is noted that the fourth step may be taken before the third step, soas to first reach the anchor location P₂ at larger radius and then reachthe anchor location P₁ at smaller radius.

In the above, the tilt θ is expressed in terms of displacement Δz andΔψ. Normally, the control unit 90 does not have direct informationregarding these parameters. Of course, it is possible to provide thecontrol unit with sensors for measuring Δz and Δψ, respectively, butthis is not preferred since it involves additional hardware and costs.

However, the control unit 90 has signals available representing saidparameters Δz and Δψ. Generally, the displacement Δz and Δψ establishedby the respective actuators is proportional to the respective controlsignal generated by the control unit 90, especially if saiddisplacements are small. In those cases, the following relationshipsapply:Δz=γ _(Z) ·S _(CPa);Δψ=γ_(ψ) ·S _(CPp)wherein γ_(Z), γ_(ψ)· are proportionality constants. Thus, the controlunit 90 can calculate said displacements from its own control signals.

In the above, it is explained that the tilt θ(r,φ) in one locationP(r,φ) can be determined if the X-coordinates and the Z-coordinates ofanchor points P1 and P2 are known. In principle, such measurements couldbe performed for one individual measuring point, keeping the opticaldisc 2 stationary. Normally, however, it is desired to determine thetilt θ(r,φ) for a large number of values of r and φ, i.e. over a largenumber of locations (or perhaps even over entire surface of the opticaldisc). This could be done by repeating the measuring method for eachsuch location. However, this would be very impractical.

Therefore, in practice, the measuring method proposed by the presentinvention is preferably implemented by determining the tilt θ(R_(i),φ)over 360° at a certain radius R_(i), i.e. for all points P_(j) at acertain radius R_(i), with a rotating disc. First, the objective lens isbrought to an initial focus location at a certain radius R_(i),corresponding to a certain X-coordinate. Then, for all anchor pointsP_(1j)(R_(i)−Δr,φ_(j)) at lower radius, the X-coordinatesX_(1j)(R_(i)−Δr,φ_(j)) and the Z-coordinates Z_(1j)(R_(i)−Δr,φ_(j)) aredetermined and stored in a memory. Subsequently, for all anchor pointsP_(2j)(R_(i)+Δr,φ_(j)) at larger radius, the X-coordinatesX_(2j)(R_(i)+Δr,φ_(j)) and the Z-coordinates Z_(2j)(R_(i)+Δr,φ_(j)) aredetermined and stored in a memory. Of course, the order may be reversed.Then, for each point P_(j)(R_(i),φ_(j)) at this radius R_(i), the tiltθ_(j)(R_(i),φ_(j)) can be calculated by combining the coordinatesX_(1j)(R_(i)−Δr,φ_(j)), X_(2j)(R_(i)+Δr,φ_(j)), Z_(1j)(R_(i)−Δr,φ_(j)),Z_(2j)(R_(i)+Δr,φ_(j)) in the manner described earlier.

This can be repeated for different values of R_(i), in order to obtainthe tilt over the entire disc.

More particularly, the method will be explained in somewhat more detailfor each of the above-described embodiments.

Referring to the first implementation explained above with reference toFIG. 3, and to the flow chart of FIG. 5A, the optical disc 2 is rotated(step 501) at a certain predetermined speed. Then, in a first step 502,the objective lens 34 is brought to an initial focus position (34 ¹)(x₀,0,z₀,ψ₀) corresponding to measuring radius Ri.

In a second step 503, the pivot actuator 41 is activated such as topivot the objective lens 34 over a first angle Δψ₁ towards smallerradius, to reach a position (34 ²) having coordinates (x₀,0,z₀,ψ₀−ψ₁).In a third step 504, the focal actuator 37 is activated such as toregain focus (34 ³) and to maintain focus. The focal control signal issampled (step 505) over at least one revolution of the optical disc 2,and the sampled values are stored in a memory, in correlation to theangular position at which the focal control signal was sampled.

In a fourth step 506, the objective lens 34 is brought back to theinitial focus position (x₀,0,z₀,ψ₀).

In a fifth step 507, the pivot actuator 41 is activated such as to pivotthe objective lens 34 over a second angle Δψ₂ towards larger radius, toreach a position (34 ⁴) having coordinates (x₀,0,z₀,ψ₀+Δψ₂). In a sixthstep 508, the focal actuator 37 is activated such as to regain focus (34⁵) and to maintain focus. The focal control signal is sampled (step 509)over at least one revolution of the optical disc 2, and the sampledvalues are stored in a memory, in correlation to the angular position atwhich the focal control signal was sampled.

Along a circle at radius Ri, the tilt θ_(j)(Ri,φ_(j)) can now becalculated (step 510) for different angular coordinates φ_(j).

Referring to the second implementation explained above with reference toFIG. 4, and to the flow chart of FIG. 5B, the optical disc 2 is rotated(step 521) at a certain predetermined speed. Then, in a first step 522,the objective lens 34 is brought to an initial focus position (34 ¹)(x₀,0,z₀,ψ₀) corresponding to measuring radius Ri.

In a second step 523, the focal actuator 37 is activated such as toaxially displace the objective lens 34 over a distance Δz towards thedisc, to reach a position (34 ²) having coordinates (x₀,0,z₀+Δz,ψ₀). Bysuch step, the beam is now out of focus. In a third step 524, the pivotactuator 41 is activated such as to pivot the objective lens 34 towardssmaller radius, to regain focus (34 ³), and to maintain focus. The pivotcontrol signal is sampled (step 525) over at least one revolution of theoptical disc 2, and the sampled values are stored in a memory, incorrelation to the angular position at which the pivot control signalwas sampled.

In a fourth step 526, the pivot actuator 41 is activated such as topivot the objective lens 34 towards larger radius, to regain focus (34⁴), and to maintain focus. The pivot control signal is sampled (step527) over at least one revolution of the optical disc 2, and the sampledvalues are stored in a memory, in correlation to the angular position atwhich the pivot control signal was sampled.

Along a circle at radius Ri, the tilt θ_(j)(Ri,φ_(j)) can now becalculated (step 530) for different angular coordinates φ_(j).

Usually, the objective lens 34 is a symmetrical lens, and usually, inthe case of a symmetrical lens, the tilt correction is optimal if theoptical axis or main axis of the lens is directed substantiallyperpendicular to the reflecting layer of the disc, i.e. if the opticalaxis of the lens makes a pivot angle ψ equal to the tilt θ of the disc2. For such cases, tilt correction can be achieved without needing toknow the magnitude of the tilt. Likewise, for such cases it isrelatively easily possible to check whether the objective lens has anadequate tilt correction position without needing to know the magnitudeof the tilt and even without needing to know the current magnitude ofthe pivot angle.

In the above discussions with reference to FIGS. 3 and 4, and FIGS.5A-B, the initial pivot angle in the initial focus position of theobjective lens has been indicated as ψ₀. Starting from a normalsituation, the initial pivot angle ψ₀ will normally be zero, asillustrated in FIGS. 3 and 4. However, the value of the initial pivotangle ψ₀ is not of great importance when calculating the tilt angle θ inaccordance with the method proposed by the invention and discussedabove.

On the other hand, if the initial pivot angle ψ₀ is equal to the tiltangle θ, the pivot angles Δψ₁ and Δψ₂ are equal to each other (see FIG.4), or the axial displacements ΔZ₁ and ΔZ₂ are equal to each other ifthe pivot angles Δψ₁ and Δψ₂ are chosen equal to each other (see FIG.3), as can easily be seen.

Based on this recognition, the present invention also proposes a methodfor setting the objective lens 34 at an adequate pivot angle suitablefor correcting tilt, as will be explained with reference to FIGS. 6A-B.It is noted that the disc may be rotated, but may also be leftstationary.

First, a pivot offset ψ₀ is selected (step 601), and the objective lens34 is brought to a focus condition (step 602). Then, as discussed withreference to FIG. 3, the objective lens 34 is pivoted (step 603) over afirst angle Δψ₁ and axially displaced (step 604) over a first axialdistance ΔZ₁ such as to regain focus, and this first axial distance ΔZ₁is stored (step 605) in a memory. Subsequently the objective lens 34 ispivoted (step 607) over a second angle Δψ₂ equal but opposite to thefirst angle Δψ₁, and axially displaced (step 608) over a second axialdistance ΔZ₂ such as to regain focus, and this second axial distance ΔZ₂is stored (step 609) in a memory. These two axial distances ΔZ₁ and ΔZ₂are compared (step 610) to each other. If the pivot offset ψ₀corresponds to the tilt, said two axial distances ΔZ₁ and ΔZ₂ will beequal to each other. If said two axial distances ΔZ₁ and ΔZ₂ are not,within certain limits, equal to each other, the pivot offset ψ₀ isreadjusted (step 611), and the above steps 602-609 are repeated untilsaid two axial distances ΔZ₁ and ΔZ₂ are substantially equal to eachother. Then, the current value of the pivot offset ψ₀ will be taken asoperational pivot angle ψ_(C) (step 612). It is noted that in thismethod it is not necessary to calculate θ, and it is not necessary toknow the focal distance f.

An alternative procedure, equally advantageous, is proposed on the basisof the method discussed with reference to FIGS. 4 and 5B. First, a pivotoffset ψ₀ is selected (step 621), and the objective lens is brought to afocus condition (step 622). Then, the objective lens 34 is axiallydisplaced (step 623) over a certain axial distance ΔZ. The objectivelens 34 is pivoted (step 624) over a first angle Δψ₁ such as to regainfocus, and this first angle Δψ₁ is stored (step 625) in a memory.Subsequently, the objective lens 34 is pivoted (step 626) over a secondangle Δψ₂ opposite to the first angle Δψ₁, such as to regain focus, andthis second angle Δψ₂ is stored (step 627) in a memory. These two axialangles Δψ₁ and Δψ₂ are compared (step 630) to each other. If the pivotoffset ψ₀ corresponds to the tilt, said two angles Δψ₁ and Δψ₂ will beequal to each other. If said two axial angles Δψ₁ and Δψ₂ are not,within certain limits, equal to each other, the pivot offset ψ₀ isreadjusted (step 631), and the above steps 623-627 are repeated untilsaid two angles Δψ₁ and Δψ₂ are substantially equal to each other. Then,the current value of the pivot offset ψ₀ will be taken as operationalpivot angle ψ_(C) (step 632). It is noted that in this method, too, itis not necessary to calculate θ, and it is not necessary to know thefocal distance f.

Also, during operation of an optical disc drive apparatus, it may bethat the signal quality deteriorates. One of the possible causes may bethat the disc has a tilt which does not correspond to the tiltcorrection setting of the objective lens. According to the presentinvention, this can be checked relatively easily. The current pivotangle of the objective lens is taken as initial pivot angle ψ₀, and theabove-discussed steps 603-609 or 623-627, respectively, are taken. Theresult of step 610 or 630, respectively, determines whether the currentpivot angle corresponds to an adequate tilt correction, or, conversely,whether the disc has obtained a tilt deviating from the current pivotangle. If necessary, the pivot angle is adjusted (step 611 or 631,respectively), and the pivot angle setting procedure discussed above isfollowed.

An important advantage of this method is that the radial coordinate x₀of the objective lens is maintained.

As mentioned above, usually, the objective lens 34 is positioned foroptimal tilt correction if the optical axis or main axis of the lens isdirected substantially perpendicular to the reflecting layer of thedisc, i.e. if the operational pivot angle ψ_(C) is equal to the tilt θof the disc 2. However, this is not necessarily always true. It may bethat, for optimal tilt correction, the operational pivot angle ψ_(C)should be different from the tilt θ of the disc 2. Whether this will bethe case or not, depends on the type of lens, and is known in advance.Further, it will be possible for the manufacturer of the disc driveapparatus to determine in advance an optimal relationship between theoperational pivot angle ψ_(C) and the tilt θ of the disc 2. Thisrelationship can be stored in a memory associated with the control unit90, for instance in the form of a look-up table.

Then, after the tilt θ has been determined by any of the above-mentionedmethods, the control unit 90 may set the operational pivot angle ψ_(C)of the objective lens 34 in accordance with the relationship stored insaid memory.

In the situation discussed above where it is desired to check whetherthe disc has a tilt which does not correspond to the tilt correctionsetting of the objective lens, for instance because the signal qualityhas deteriorated, the initial pivot angle ψ₀ is set on the basis of thecurrent pivot angle of the objective lens taking said relationshipstored in said memory into account.

It should be clear to a person skilled in the art that the presentinvention is not limited to the exemplary embodiments discussed above,but that various variations and modifications are possible within theprotective scope of the invention as defined in the appending claims.

For instance, it is possible that the laser 31 and the detector 35 aremounted with respect to the frame 3.

1. In an optical disc drive apparatus, of a type comprising: rotatingmeans defining a rotating axis for an optical disc; optical scanningmeans for scanning an optical disc with a light beam, said opticalscanning means comprising a displaceable objective lens for focussingthe light beam onto said optical disc, said objective lens beingdisplaceable in axial direction and capable of being pivoted about anaxis directed in tangential direction; a method for measuring tilt in ameasuring location of the optical disc; the method comprising the stepsof: by pivoting and axially displacing the objective lens, bringing saidobjective lens to a first focus measuring location such as to focus thelight beam in a first anchor point having substantially the same angularcoordinate φ as said measuring location and having a small radialdistance Δr1 from said measuring location; by displacing and pivotingthe objective lens, bringing said objective lens to a second focusmeasuring location such as to focus the light beam in a second anchorpoint having substantially the same angular coordinate φ as saidmeasuring location and having a small radial distance Δr2 from saidmeasuring location; said first and second anchor points being located onopposite sides of said measuring location; the method further comprisingthe step of calculating tilt in said measuring location from thecoordinates of said two focus measuring locations of said objectivelens.
 2. Method according to claim 1, comprising the steps of: bringingthe objective lens to an initial focus position such as to focus thelight beam in said measuring location; with respect to said initialfocus position, pivoting the objective lens over a first angle towardssmaller radius; displacing the objective lens axially over a first axialdistance such that the optical beam is again focused on the disc; withrespect to said initial focus position, pivoting the objective lens overa second angle towards larger radius; displacing the objective lensaxially over a second axial distance such that the optical beam is againfocused on the disc.
 3. Method according to claim 2, wherein tilt of themeasuring location is calculated in accordance with the formula:tan θ(r,φ)=(f·cos(Δψ₁)+Δz ₁−(f·cos(Δψ₂)−Δz ₂))/(f·sin(Δψ₁)+f·sin(Δψ₂))4. Method according to claim 2, wherein the first angle is equal to thesecond angle.
 5. Method according to claim 1, comprising the steps of:bringing the objective lens to an initial focus position such as tofocus the light beam in said measuring location; with respect to saidinitial focus position, axially displacing the objective lens over anaxial distance towards the disc; pivoting the objective lens over afirst pivot angle towards smaller radius such that the optical beam isagain focused on the disc; pivoting the objective lens over a secondpivot angle towards larger radius such that the optical beam is againfocused on the disc.
 6. Method according to claim 5, wherein tilt of themeasuring location is calculated in accordance with the formula:tan θ(r,φ)=(cos(Δψ₁)−cos(Δψ₂))/(sin(Δψ₁)+sin(Δψ₂))
 7. Method accordingto claim 1, wherein the measurements are performed while the disc isbeing rotated, such that measurement results are obtained for aplurality of points located at a first radius, and these measurementresults are stored in a memory in correlation to the correspondingangular coordinate; measurement results are obtained for a plurality ofpoints located at a second radius, and these measurement results arestored in a memory in correlation to the corresponding angularcoordinate; and the tilt at at least one location at an intermediateradius and having a certain angular coordinate is calculated from themeasurement results stored in said memories.
 8. Optical disc driveapparatus, comprising: rotating means defining a rotating axis for anoptical disc; optical scanning means for scanning an optical disc with alight beam, said optical scanning means comprising: a light beamgenerating means for generating a light beam; a displaceable objectivelens for focussing the light beam onto said optical disc; the apparatusfurther comprising: radial actuator means for radially displacing saidobjective lens; axial actuator means for axially displacing saidobjective lens; pivot actuator means for pivoting said objective lens;control means for controlling said radial actuator means, said axialactuator means, and said pivot actuator means; said control means beingdesigned for measuring tilt in a measuring location of an optical discby: by pivoting and axially displacing the objective lens, bringing saidobjective lens to a first focus measuring location such as to focus thelight beam in a first anchor point having substantially the same angularcoordinate φ as said measuring location and having a small radialdistance Δr1 from said measuring location; by pivoting and axiallydisplacing the objective lens, bringing said objective lens to a secondfocus measuring location such as to focus the light beam in a secondanchor point having substantially the same angular coordinate φ as saidmeasuring location and having a small radial distance Δr2 from saidmeasuring location, said first and second anchor points being located onopposite sides of said measuring location; calculating tilt in saidmeasuring location from the coordinates of said two focus measuringlocations of said objective lens.
 9. Disc drive apparatus according toclaim 8, wherein said control means is designed to: activate said radialactuator means and said axial actuator means in order to bring theobjective lens to an initial focus position such as to focus the lightbeam in said measuring location; activate said pivot actuator means inorder to pivot the objective lens over a first pivot angle towardssmaller radius with respect to said initial focus position; activatesaid axial actuator means in order to axially displace the objectivelens over a first axial distance such that the optical beam is againfocused on the disc; activate said pivot actuator means in order topivot the objective lens over a second pivot angle towards larger radiuswith respect to said initial focus position; activate said axialactuator means in order to axially displace the objective lens over asecond axial distance such that the optical beam is again focused on thedisc.
 10. Apparatus according to claim 9, wherein said control means isdesigned to calculate tilt of the measuring location in accordance withthe formula:tan θ(r,φ)=(f·cos(Δψ₁)+Δz ₁−(f·cos(Δψ₂)−Δz ₂))/(f·sin(Δψ₁)+f·sin(Δψ₂))11. Apparatus according to claim 9, wherein the first angle is equal tothe second angle.
 12. Disc drive apparatus according to claim 8, whereinsaid control means is designed to: activate said radial actuator meansand said axial actuator means in order to bring the objective lens to aninitial focus position such as to focus the light beam in said measuringlocation; activate said axial actuator means in order to axiallydisplace the objective lens over an axial distance towards the disc;activate said pivot actuator means in order to pivot the objective lensover a first pivot angle towards smaller radius such that the opticalbeam is again focused on the disc; activate said pivot actuator means inorder to pivot the objective lens over a second pivot angle towardslarger radius such that the optical beam is again focused on the disc.13. Disc drive apparatus according to claim 12, wherein said controlmeans is designed to calculate tilt of the measuring location inaccordance with the formula:tan θ(r,φ)=(cos(Δψ₁)−cos(Δψ₂))/(sin(Δψ₁)+sin(Δψ₂))
 14. Apparatusaccording to claim 8, wherein the control unit is designed to: activatethe rotating means such as to rotate the disc; activate said radialactuator means in order to bring the objective lens to an initial radialposition; activate said axial actuator means in order to bring theobjective lens to an initial focus position; activate said pivotactuator such as to pivot the objective lens over a first angle towardssmaller radius; activate said axial actuator means in order to obtainand maintain a focus condition; sample the focal control signal over atleast one revolution of the optical disc; store the sampled values in amemory, in correlation to the angular position at which the focalcontrol signal was sampled; activate said pivot actuator such as topivot the objective lens over a second angle towards larger radius;activate said axial actuator means in order to obtain and maintain afocus condition; sample the focal control signal over at least onerevolution of the optical disc; store the sampled values in a memory, incorrelation to the angular position at which the focal control signalwas sampled; calculate the tilt at a location at said initial radius,using the stored values.
 15. Apparatus according to claim 8, wherein thecontrol unit is designed to: activate the rotating means such as torotate the disc; activate said radial actuator means in order to bringthe objective lens to an initial radial position; activate said axialactuator means in order to bring the objective lens to an initial focusposition; activate said axial actuator means in order to axiallydisplace the objective lens over a distance towards the disc; activatesaid pivot actuator in a first direction in order to obtain and maintaina focus condition; sample the pivot control signal over at least onerevolution of the optical disc; store the sampled values in a memory, incorrelation to the angular position at which the pivot control signalwas sampled; activate said pivot actuator in a second direction in orderto obtain and maintain a focus condition; sample the pivot controlsignal over at least one revolution of the optical disc; store thesampled values in a memory, in correlation to the angular position atwhich the pivot control signal was sampled; calculate the tilt at alocation at said initial radius, using the stored values.
 16. In anoptical disc drive apparatus, of a type comprising: rotating meansdefining a rotating axis for an optical disc; optical scanning means forscanning an optical disc with a light beam, said optical scanning meanscomprising a displaceable objective lens for focussing the light beamonto said optical disc, said objective lens being displaceable in axialdirection and capable of being pivoted about an axis directed intangential direction; a method for setting an operational pivot angle ofthe objective lens; the method comprising the steps of: [a] selecting aninitial pivot offset; [b] bringing the objective lens to an initialfocus position (x₀, 0, z₀, ψ₀); [c] with respect to said initial focusposition (x₀, 0, z₀, ψ₀), pivoting the objective lens over a first angletowards smaller radius to a position (x₀, 0, z₀, ψ₀−Δψ₁); [d] displacingthe objective lens axially over a first axial distance such that theoptical beam is again focused on the disc; [e] with respect to saidinitial focus position (x₀, 0, z₀, ψ₀), pivoting the objective lens overa second angle towards larger radius to a position (x₀, 0, z₀, ψ₀+Δψ₂),wherein the second angle is equal to said first angle; [f] displacingthe objective lens axially over a second axial distance such that theoptical beam is again focused on the disc; [g] comparing said firstaxial distance with said second axial distance; [h1] if said first axialdistance is not, within a certain limit, substantially equal to saidsecond axial distance, readjust the pivot offset and repeat steps[b]-[g]; [h2] if said first axial distance is substantially equal tosaid second axial distance, set the operational pivot angle of theobjective lens on the basis of the current value of the pivot offset.17. In an optical disc drive apparatus, of a type comprising: rotatingmeans defining a rotating axis for an optical disc; optical scanningmeans for scanning an optical disc with a light beam, said opticalscanning means comprising a displaceable objective lens for focussingthe light beam onto said optical disc, said objective lens beingdisplaceable in axial direction and capable of being pivoted about anaxis directed in tangential direction; a method for setting anoperational pivot angle of the objective lens; the method comprising thesteps of: [a] selecting an initial pivot offset; [b] bringing theobjective lens to an initial focus position (x₀, 0, z₀, ψ₀); [c] withrespect to said initial focus position (x₀, 0, z₀, ψ₀), axiallydisplacing the objective lens over an axial distance towards the disc;[d] pivoting the objective lens over a first pivot angle towards smallerradius to a position (x₀, 0, z₀, ψ₀−Δψ₁), such that the optical beam isagain focused on the disc; [e] pivoting the objective lens over a secondpivot angle towards larger radius to a position (x₀, 0, z₀, ψ₀+Δψ₂) suchthat the optical beam is again focused on the disc; [f] comparing saidfirst pivot angle with said second pivot angle; [g1] if said first pivotangle is not, within a certain limit, substantially equal to said secondpivot angle, readjust the pivot offset and repeat steps [b]-[f]; [g2] ifsaid first pivot angle is substantially equal to said second pivotangle, set the operational pivot angle of the objective lens on thebasis of the current value of the pivot offset.
 18. Method according toclaim 16, wherein the operational pivot angle of the objective lens isset to be equal to the current value of the pivot offset.
 19. Methodaccording to claim 16, wherein the operational pivot angle of theobjective lens is set on the further basis of an optimal relationshipbetween the operational pivot angle and the tilt of the disc. 20.Optical disc drive apparatus, comprising: rotating means defining arotating axis for an optical disc; optical scanning means for scanningan optical disc with a light beam, said optical scanning meanscomprising: a light beam generating means for generating a light beam; adisplaceable objective lens for focussing the light beam onto saidoptical disc; the apparatus further comprising: radial actuator meansfor radially displacing said objective lens; axial actuator means foraxially displacing said objective lens; pivot actuator means forpivoting said objective lens; control means for controlling said radialactuator means, said axial actuator means, and said pivot actuatormeans; said control means being designed for setting an operationalpivot angle of the objective lens by: [a] selecting an initial pivotoffset; [b] activating said radial actuator means and said axialactuator means in order to bring the objective lens to an initial focusposition (x₀, 0, z₀, ψ₀); [c] activating said pivot actuator means inorder to pivot the objective lens over a first pivot angle towardssmaller radius to a position (x₀, 0, z₀, ψ₀−Δψ₁); [d] activating saidaxial actuator means in order to axially displace the objective lensover a first axial distance such that the optical beam is again focusedon the disc; [e] activating said pivot actuator means in order to pivotthe objective lens over a second pivot angle towards larger radius to aposition (x₀, 0, z₀, ψ₀+Δψ₂), wherein the second angle is equal to saidfirst angle; [f] activating said axial actuator means in order toaxially displace the objective lens over a second axial distance suchthat the optical beam is again focused on the disc [g] comparing saidfirst axial distance with said second axial distance; [h1] if said firstaxial distance is not, within a certain limit, substantially equal tosaid second axial distance, readjust the pivot offset and repeat steps[b]-[g]; [h2] if said first axial distance is substantially equal tosaid second axial distance, set the operational pivot angle of theobjective lens on the basis of the current value of the pivot offset.21. Optical disc drive apparatus, comprising: rotating means defining arotating axis for an optical disc; optical scanning means for scanningan optical disc with a light beam, said optical scanning meanscomprising: a light beam generating means for generating a light beam; adisplaceable objective lens for focussing the light beam onto saidoptical disc; the apparatus further comprising: radial actuator meansfor radially displacing said objective lens; axial actuator means foraxially displacing said objective lens; pivot actuator means forpivoting said objective lens; control means for controlling said radialactuator means, said axial actuator means, and said pivot actuatormeans; said control means being designed for setting an operationalpivot angle of the objective lens by: [a] selecting an initial pivotoffset; [b] activating said radial actuator means and said axialactuator means in order to bring the objective lens to an initial focusposition (x₀, 0, z₀, ψ₀); [c] activate said axial actuator means inorder to axially displace the objective lens over an axial distancetowards the disc; [d] activate said pivot actuator means in order topivot the objective lens over a first pivot angle towards smaller radiusto a position (x₀, 0, z₀, ψ₀−Δψ₁), such that the optical beam is againfocused on the disc; [e] activate said pivot actuator means in order topivot the objective lens over a second pivot angle towards larger radiusto a position (x₀, 0, z₀, ψ₀+Δψ₂) such that the optical beam is againfocused on the disc; [f] comparing said first pivot angle with saidsecond pivot angle; [g1] if said first pivot angle is not, within acertain limit, substantially equal to said second pivot angle, readjustthe pivot offset and repeat steps [b]-[f]; [g2] if said first pivotangle is substantially equal to said second pivot angle, set theoperational pivot angle of the objective lens on the basis of thecurrent value of the pivot offset.